Generally, in the bicycle, a plurality of front chain gears and rear chain gears are used for the multistage speed change as is well-known; the center of the axial arrangement of the rear chain gears and the same of the front chain gears are aligned longitudinally of the bicycle. For example, in the ten stage speed change device as shown in FIGS. 10 and 11, the center of the axial arrangement of rear chain gears RG.sub.1, RG.sub.2, RG.sub.3, RG.sub.4 and RG.sub.5, that is, the gear RG.sub.3, and the center of the axial arrangement of the front chain gears FG.sub.1 and FG.sub.2, are aligned in parallel to the longitudinal center line X of the bicycle.
In the multistage speed-change device constructed as described above, the driving chain C, when engaged with the larger diameter top front chain gear FG.sub.1 and when shifted to the largest diameter top rear chain gear RG.sub.5 or the smallest diameter low rear chain gear RG.sub.1, is slanted as shown by the chain line in FIG. 11 with respect to the center line X, which is similar to shifting of chain C in engagement with the smaller diameter low front gear FG.sub.2.
Hence, a chain guide 7 at the conventional front derailleur FD should have an interval between an inner plate 8 and an outer plate 9 sufficiently larger than a width of chain C, because a narrow interval will cause the chain to incline along the chain line in FIG. 11 to come into contact with the inner plate 8 or outer plate 9, thereby generating noises and promoting wearing of each plate 8 or 9.
However, a sufficiently wide interval between both the plates 8 and 9 to satisfiy the above criteria will result in deterioration of the speed change efficiency.
Furthermore, the chain C is shifted by being pushed by the inner or outer plate in contact with the chain C, and both the plates each have a flat surface normally parallel to each front gear, thereby generating noises. Particularly, in a case where a control lever is operated slowly to pull a control wire so as to shift the chain from the smaller diameter front low gear to the larger diameter top gear, the chain is kept longer in contact with each plate, thereby elongating the noise generation time.